My 1971 Eldorado has developed a knocking noise deep in the back of the engine. Comes on when idling and when backing off the throttle but not on the road while moving. It's intermittent and happens more when the engine's cold. Car drives as well as it ever used to and changes gears fine (automatic). Anyone had this before and know the solution? I'm in the UK so not so many places to turn to than I had in the US of A.
Thanks

 

"Deep in the back"?

First Guess: Cracked flexplate.
Second guess: Rod knock.

Can you make it go away at idle by grounding the spark to a particular cylinder? Don't just pull the plug wire off, use a jumper wire or test light to ground the spark.